High speed steel is unsurpassed by other tool steels for its hot hardness. It is a preferred material for a variety of cutting and forming operations. Applications are as bits, end mills, drills, cutters, reamers, dies, shearing blades, and others. Another widely used tool material is Co-cemented tungsten carbide. High speed steel is superior to cemented carbide in ductility even when fully hardened, but is inferior in hardness. Such a high speed steel is called for that has properties intermediate between those of conventional high speed steel and of cemented carbide.
High speed steel comprises a matrix of martensite with a fine dispersion of M.sub.6 C, M.sub.23 C.sub.6, MC type carbides (M denotes a metal(s) or an alloy(s)), wherein the ductility is prescribed primarily by the properties of the matrix and the hardness by the carbide contents. It has W, Mo, Cr, V, Co, C and bal Fe as main constituents with a nominal composition of W+2Mo (W-equivalent) 10-24% (by weight), Cr 4%, V 1-5%, Co 0-17%, Mn+Si less than 2%, and the remainder C and Fe (C is usually computed from C=0.19+0.017W-equivalent+0.2-0.22V(%)), wherein W and Mo are main M.sub.6 C carbide formers, Cr is a main M.sub.23 C.sub.6 carbide former, and V a main MC carbide former (believed to exist as VC or V.sub.4 C.sub.3 in steel), the total carbide content falling in the range of 20 to 30%.
Futile attempts have been made repeatedly to increase the carbide content. Increasing M.sub.6 C carbide by increasing W-equivalent beyond the range stated above (and also carbon) on one hand is accompanied by a rapid fall of ductility with deterioration of microstructure. Increasing MC carbide by increasing vanadium (and also carbon) on the other hand in hindered by the difficulty of melting, that is, concurrent rise in the melting temperature and widening of the solid-liquid range. In addition, billets with increased carbide, in particular, with vanadium in excess of 5%, are susceptible to fracture when hot forged for fractioning coarse carbide nets formed along grain boundaries upon solidification.
In a recently proposed and commercially established atomizing technique, a molten alloy jet is cooled, at rates fast enough to suppress the formation of coarse carbide, into droplets which are then compacted in a capsule either by hot forging or by hot isostatic pressing to obtain solid billets. This process has the advantage of dispensing with the above forging step, but is still subject to limitations resulting from atomizing a vanadium-rich melt and deforming the billets into small sizes, thus the permissible vanadium content in no way exceeds 6.5%.
The present invention is based on the recognition that, while vanadium carbide once incorporated in the matrix acts as an ideal strengthener, little influenced by the existence of other carbides and the composition of the matrix, its incorporation is hindered in the conventional methods because they all start with a molten alloy melt. A method that relies solely on solid state reactions will now be disclosed, which enables as much vanadium as desired to be incorporated and thus provides a vanadium-rich high speed steel with increased hardness and least decreased ductility.